1. Field of Invention
The present invention relates to a method of hiding an electronic signature and, in particular, to a method of producing an invisible electronic signature.
2. Related Art
Owing to the rapid development in computer network technology, the whole world has entered the digital era. This has made information communications and knowledge sharing much easier. However, the issue of security over the network gradually threatens the efforts in protecting network intellectual properties. The applications of computer networks in national defense, such as the information systems in command, management, communications and intelligence or modem weapon equipment, are more and more popular. Therefore, how classified information can acquire safety authenticates on the network has become an urgent problem to be solved. Practically speaking, the so-called electronic signature technology refers to the techniques that prevent unintended users from eavesdropping, copying and modifying others"" information so that the legal user can extract authenticated information from a meaningful image. The researches of electronic signatures in digital copyright focus at ensuring that the attached electronic signature does not alter the visual quality of the original article and is not easy to be detected and changed or removed by hackers so that the belonging of the digital copyright can be correctly determined.
By electronic signature technology refers to embedding a portion of meaningful texts, image or video signal during the image transmission. It is made to be hard for an unauthorized third party to see whether there is hidden information or not in the transmission process. This can prevent hackers from damaging the transmitted information. A common method seen in the electronic signature technology is to select a host image, hide therein some information and thus generate another watermarked image. This image hidden with a signature looks just like the host image by naked eyes, and it is hard to find out directly that other information is contained therein. Other users on the network cannot determine whether this signed image contains other information or not. Thus, from all the information on the network unauthorized users are unable to distinguish electronically signed images from unsigned ones. However, a legal user can readily extract the hidden electronic signature from the transmitted information.
Normally, the electronic signature technology must have the following features:
1. Undetectable: The electronic signature is hidden behind the image information so that it would not be found using usual image processing methods.
2. Invisible: An image attached with the electronic signature looks the same as the host image by naked eyes.
3. Undeletable: The electrical signature added to the image cannot be easily deleted using simple image processing methods.
4. Resistant to image manipulation: The electrical signature is not susceptible to damages caused by normal image processing or on purpose.
The electronic signature technology has the space and frequency domains. In the research field of the frequency domain, using frequency expansions on the electronic signature is fine but has the following three disadvantages:
1. The computation is too tedious. Conversion to the frequency domain requires complicated calculation. The receiver end also needs the corresponding converter. This is inconvenient to applications that demand real time processing.
2. It is vulnerable to attacks. Most hiding methods in the frequency domain hide the information in peripheral sections to avoid damages caused by anamorphous compression. Therefore, it is easy for invaders to attack the hidden information.
3. It can hide relatively little information. The information can only be hidden within a specific frequency band in the frequency domain, so it can hide relatively little information.
In the space domain, a commonly employed information hiding method is the vector quantization. It uses a codebook commonly owned by both the sender and the receiver to encode the electronic signature. Over the sender end, the sender cuts the electronic signature to be hidden into the same size as the blocks in the code book, finds a similar block from the code book and hides in the image its index in place of the information in this block, and finally sends it to the network. Over the receiver end, when the receiver receives the image with hidden information he decodes the index, looks it up in the codebook and restores the information. The defect of this method is that there is larger information anamorphosis and that the image with hidden information cannot withstand the damages caused by anamorphosis.
Another electronic signature technology uses fixed areas in image pixels to hide information, e.g., the fixed range equalization method. The gray scale of the image to hide information in this method is divided into 16 sections. The hidden information, in unit of bytes, replaces the original value at a point depending upon the space the input value belongs to. For example, if the information has a value of 17, then one point within the range from 16xcx9c31 is thus replaced by 17. There is yet another method called LSB, which is also often used for hiding information. LSB places the electronic signature in the lower (less important) bytes because the change in lower bytes has less impact on the whole pixel values and thus has less obvious damages to the image. Nevertheless, the above two methods cannot withstand the anamorphous damages after the image is hidden with information.
The present invention provides a method of producing an invisible electronic signature, which method uses several image processing techniques. First, the B channel in the RGB color image is selected to hide information because human eyes are not sensitive to the B channel. The information is then embedded therein using cross encoding, channel encoding and pixel location correlation. With the encoding/decoding techniques and the comparison among neighboring pixels, the hidden information can be effectively protected from illegal modification. Also, because of the restoration ability of the channel encoding and the comparison among neighboring pixels, the present invention can correctly restore the information even after anamorphous compression.
The disclosed method can protect the information and withstand anamorphous damages. Using the channel encoding technique and the image correlations, a method of producing an electronic signature is provided by further combining with some basic image hiding techniques. This invention has practical values in determining the belonging of copyrights and information camouflage.
The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon a reading of the following detailed description together with the included drawings.